BACKGROUND-AIM:
Smoking initiation most commonly occurs during adolescence, a period when the lungs are still developing, which may lead to long-term adverse effects. However, epidemiological studies examining the age of smoking debut in relation to respiratory symptoms and asthma development in young adulthood remain surprisingly scarce. Thus, the aim was to prospectively study age of smoking debut and packyears in relation to respiratory symptoms and asthma at age 28 years.

METHODS:
The epidemiological research programme Obstructive Lung Disease in Northern Sweden (OLIN) studies recruited a cohort of 8-year-old children in 1996 (n=3430; 97% of invited). The cohort was followed annually until age 19 years, by questionnaire surveys about asthma, respiratory symptoms, and from age 13 years, smoking habits. Additionally, the cohort was followed-up by postal questionnaire at age 28 years (n=2291; 71% of invited). The association between smoking during adolescence and respiratory symptoms and asthma at the age of 28 years was analysed by unadjusted and adjusted logistic regression and the results presented as odds ratios (OR) with 95% confidence intervals (CI).

RESULTS:
Being a daily smoker was reported at least once by 22%, more commonly by women than men, 25 vs. 19%, p<0,001. Of ever smokers, 29% started smoking ≤15 years (mean packyear 2.3), 35% between 16 and 17 years (mean packyears: 1.8), and 35% ≥18 years (mean packyears: 1.2). Mean age of smoking debut was 16.8 years (min 10, max 26y) and mean packyears was 1.7 (min 0, max 19). Among all 2291 participants at age 28 years, 23% reported asthma and 53% reported any respiratory symptom, the most common were any wheeze last 12 months (36%) and sputum production (30%). The prevalence of asthma increased with earlier age of smoking debut: ≤15 years 26%; 16-17 years 20%, ≥18 years 20%, and 17% among never smokers, p=0.027. Similarly for any respiratory symptom: ≤15 years 46%; 16-17 years 45%, ≥18 years 40%, and 27% among never smokers, p<0.001. After adjustment for sex, family history of asthma, exposure to smoking in childhood and educational level, any respiratory symptom was associated with packyears (OR1.1 95% CI 1.0-1.2) and smoking debut age ≤15 years (OR 1.8; 95% CI 1.2-2.7), 16-17 years (OR 1.8; 95% CI 1.3-2.6), and ≥18 years (OR 1.5; 95% CI 1.1-2.1), compared with never smokers. Smoking debut ≤15 years (OR1.7; 1.2-2.6) and packyears (OR 1.1; 95% CI 1.0-1.2) was associated with asthma in unadjusted analyses, but not in the adjusted analyses.

CONCLUSIONS:
The majority of smokers started smoking before 18 years of age, and early smoking debut and higher number of packyears were associated with respiratory symptoms already at age 28 years, despite low number of packyears.